It is known that vacuum cleaners are used for household cleaning, in which the dusty air flow drawn in by a motor/suction unit passes through a dust-collecting filter bag and is purified by the latter.
The periodic replacement of the filter, necessary if the vacuum cleaner is to be able to function well (vacuum cleaners in which the filter must be periodically cleaned are now obsolete) and having to be carried out as frequently as possible in order to ensure a high level of efficiency, is a serious inconvenience for the user because handling the dirty filter during its removal and replacement results in the generation of dust.
A further drawback of these vacuum cleaners is the power consumption which increases the greater the degree of filtering required and the ability to capture finer dust particles, because the filter bag constitutes an obstacle for the air flow and causes significant head losses which increase in accordance with the increase in the accumulation of dust in the bag and clogging of the latter.
In order to achieve a higher and more constant level of efficiency as well as a greater capacity for capturing the dust, including the finest sort, which is very often the cause of allergies, cyclone-type domestic vacuum cleaners have also been proposed and introduced onto the market, in which the separation of a considerable fraction of the dust and particles of larger size and greater density is achieved, by means of the centrifugal effect, with very little loss of head, inside a tangential cyclone, while a filter arranged downstream of the cyclone is assigned the task of capturing the finest particles.
The larger-sized solid particles are accumulated in a receptacle which may be periodically emptied.
As a further step, vacuum cleaners with two tangential cyclones in a cascade arrangement have also been proposed, the first cyclone having the function of capturing the larger-sized particles, and the second the function of performing further filtering.
Examples of this solution are given in EP-A-0018197, EP-A-0489565 and EP-A-0042723.
A drawback with twin-cyclone vacuum cleaners is their large size which, in order to be reduced, generally requires that the two cyclones be arranged coaxially and, to a certain degree, inside one another.
This has the consequence that the air flow follows a winding path, with many reversals and changes in direction which result in significant head losses and require the use of a powerful motor/suction unit in order to generate a suitable air flow rate in the vacuum cleaner.
Thus, the advantage arising from the low head losses in the cyclones is lost on account of the high head losses in the ducts connecting cyclones and motor/suction unit.
The reduction in dimensions achieved is, however, of a limited nature and not sufficient to satisfy the user's demands.
There are, in fact, technical limitations which prevent reduction of the dimensions beyond a certain limit.
In cyclones, separation of the dust is caused by the combined effect of several factors:
A considerable speed must be imparted to the air flow which is introduced tangentially into a cyclone so that the centrifugal effect results in efficient separation of the solid particles, which are concentrated at the periphery of the cyclone in the vicinity of the walls.
Moreover, in the cyclone, the speed of the air flow must be reduced considerably so that the centrifugal effect and the friction exerted by the walls on the particles are able to prevail over the conveying action exerted by the air flow which increases in proportion with speed.
It is therefore obvious that, in relation to the flow rate of sucked-in air, the dimensions of the cyclones cannot be less than a certain limit, even though, from a theoretical point of view, it would be desirable to use cyclones with as small a diameter as possible, because this increases the separation efficiency (the centrifugal acceleration is defined by V.sup.2 /R where V is the tangential speed and R the radius of curvature imparted to the flow).
The particles which gradually accumulate on the walls of the cyclone, owing to the action of the force of gravity, then tend to travel towards the bottom part of the cyclone where they may be collected in a suitable storage receptacle which must be periodically emptied.
This involves a further functional limitation: the cyclones must operate with the axis of the cyclone, i.e. the axis of the frustoconical or cylindrical element which forms the cyclone, arranged vertically or nearly vertically, the flow being introduced tangentially at the top thereof, extraction of the purified air along the axis of the cyclone, far from its base, and collection of the dust at the base of the cyclone.
This limitation has serious consequences from a structural point of view when several cyclones are to be arranged in cascade. Thus the flow of sucked-in air emerging from the top of the second cyclone must in fact be conveyed towards a motor/suction unit which, owing to static phenomena and often also for functional reasons (often it must also operate the rotating brushes), is generally located beneath the cyclones, in a foot section equipped with wheels for travelling on the ground.
For this purpose, either an annular duct which surrounds the cyclones, or a connecting tube, is used.
However, both solutions reduce the amount of space available for the particle-capturing cyclones and for the storage receptacles, increasing the overall dimensions of the assembly and reducing the available storage capacity which, however, is essential for ensuring correct operation of the vacuum cleaner even when the latter is arranged in an inclined position.